Achilles tendon ruptures are common and devastating injuries, resulting in significant pain, disability, and healthcare costs. Despite the higher risk for complications and increased costs, operative treatment is often assumed to provide superior outcomes compared to non-operative treatment. However, a recent comprehensive review by the American Academy of Orthopaedic Surgeons concluded that the scientific evidence supporting current clinical paradigms, such as operative or non-operative care, and particular rehabilitation protocols, was weak and/or inconclusive. Further, a 2012 meta-analysis of randomized clinical trials concluded that outcomes for Achilles ruptures are not superior with surgical treatment, depending on the rehabilitation protocol, further calling into question the common belief of a preferred surgical treatment. The overall goal of this study is to define the tissue-level properties of healing Achilles tendon, in a targeted set of clinically-relevant conditions, that govern the mechanical mechanisms of successful tissue remodeling, and, ultimately, to relate these tissue-level properties to in vivo measures. In particular, we propose that the healing tendon environment is governed by specific structural and biologic properties which drive tendon fatigue mechanical parameters, which in turn, govern successful progression of rehabilitation. An animal model will be used to carefully control the injury and treatment strategies, rigorously analyze tissue properties, and, through our innovative methods determine clinically relevant measures which define tissue healing. Our overall hypothesis is that there are specific tissue-level properties, in clinically-relevant conditions, that govern a successful versus failed healing response of injured Achilles tendon and that these properties will not relate to currently used in vivo measures. To provide high impact and innovation, two novel aims are designed to directly challenge two commonly-accepted paradigms for Achilles tendon treatment and healing. 1: To determine whether a repaired Achilles tendon has superior tendon mechanical, structural and biological properties compared to a non-repaired tendon, and confirm the necessary measures which define successful healing for a set of six clinically relevant conditions. 2: Define the in vivo measurement techniques that are surrogates for Achilles tendon healing through our innovative ex vivo assays. As noted, this project has high impact by determining rigorous scientific and mechanistic mechanical data governing Achilles tendon healing, supporting or refuting clinically accepted paradigms regarding 1) the role of repair and specific rehabilitation paradigms and 2) the use of functional measures for return to activity criteria following Achilles tendon rupture. Furthermore, our innovative, non-invasive techniques that parallel clinical assays, coupled with our direct measures of tissue healing, such as tendon fatigue behavior from mechanical testing and collagen alignment through ultrasound, provide data to guide post-injury treatment progression and return to activity criteria in patients. Thus, this study has direct translation to clinical practice and patient care.